Giant Prehistoric Sperm. ‘Nuff Said.

Close-up of an ostracod, a crustacean that can produce giant sperm bigger than itself. Image by Anna33 at en.wikipedia.

Earlier this month, inspired by Daily Mail and HuffPo nonsense, I went on a tear about dinosaur sex. Because if you’re going to rant, why not go big? The post kicked up a good deal of attention, and I was honored to be bestowed with the highly-vaunted title of “the closest thing the Internet has to an expert on dinosaur sex” by Cracked’s Daniel O’Brien.

After that minor internet kerfuffle, I thought I had said my peace on paleo-copulation. Best to let sleeping dinosaurs lie, especially if they’ve just had sex. But after I got back from fieldwork at Dinosaur National Monument and starting riffling through journals for new subjects to write about, I saw a paper I couldn’t resist. The subject, as the title makes clear in its understated way, is “exceptionally well-preserved giant spermatozoa” discovered in prehistoric ostracods. Giant fossilized sperm? Yes, please!

“Ostracod” isn’t a household name. That’s too bad, because there are tens of thousands of species of these tiny, shrimp-like crustaceans. Typically, their segmented bodies are surrounded by a hinged, tough outer shell, and they occupy a variety of niches in both fresh and saltwater habitats the world over. In this case, the ostracod in question is Cypria ophtalmica – a freshwater species that’s still with us today.

The Cypria ophtalmica fossils aren’t very old. In fact, they barely make the cutoff for what counts as prehistoric. Discovered in the Southern Carpathian Mountains of Romania, the invertebrate fossils are only about 14,400 to 10,000 years old. But what makes them remarkable is the fact that Babes-Bolyai University paleontologist Sandra Iepure and colleagues found prehistoric sperm with both male and female ostracods, the “richest” evidence of fossil sperm yet found.

This isn’t the first time paleontologists have found evidence of ancient sperm. Previous studies have found sperm among roughly 5,000 year old ostracods from the UK, spermatophores were preserved in 40 million year old amber with an insect called a springtail, and paleontologists identified the essential seminal and receptacle organs among 100 million year old ostracods discovered in Brazil. I never thought that something as ephemeral as prehistoric sperm could survive for thousands or even millions of years, but, once again, the fossil record has shown me up. The new fossil sperm collection analyzed by Iepure and collaborators adds to the catalog of surprising fossils.

What makes the fossilized sperm so easy to spot is their size. After some fly species, ostracods have the second longest sperm in relation to their body size. As Iepure and co-authors note, some 1 millimeter long ostracods can produce sperm up to a 1 centimeter long, stretching ten times the body length of the ostracod itself. (I only pray that David Cronenberg never learns this fact.) Sadly, Iepure and co-authors don’t provide length estimates for the specimens they describe, but they note that the prehistoric specimens are consistent with what researchers have observed among living ostracods.

Ostracod sperm found in males both fossil (a,c) and recent (b,d) as seen under LSCM microscopy. From Iepure et al., 2012.

Between five and ten percent of the ostracods in the study sample contained sperm, but the individuals within this subset don’t all preserve the gametes in the same way. In most of the specimens, the sperm bundles appear to be neatly arranged in the position expected for the male reproductive organs. In the other specimens, the sperm bunches are located in a different place and are dislocated from each other. Based on observations of living ostracods, Iepure and colleagues propose that individuals with neat arrangements are males and the ones with the messier bundles are inseminated females. This is the first time prehistoric sperm has been found preserved in both sexes.

So we can add a few more amorous ostracods to the growing list of creatures that died just about the time they mated. And, of course, this raises the question of just how far the fossil record of sperm goes. The ostracods in the new study were fortuitously preserved in an environment that kept some of their delicate features intact, and the much older springtail fossil was trapped in amber. The further we go back in time, the less likely it seems that even giant sperm could have been preserved by the typical modes of burial and fossilization, but amber does offer a possible mode of preservation. Perhaps, as they continue to scour amber samples, paleontologists will discover even older prehistoric sperm. Don’t hold your breath for dinosaur semen, though. It would take a hell of a coincidence for dinosaur sperm to somehow wind up locked in amber, survive for over 66 million years, be discovered, and correctly identified by a researcher. For now, we’re left to marvel at the exceptional gametes of the little ostracods.

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